I added a comment earlier, but it hasn't yet been approved yet. So I'll double up - just in case it doesn't get approved until later.
(Bit of a lengthy reply).
After Wyorp Rock's comment on grit I double checked the chicken starter feed I had ( I was under the impression it had grit in it). It did not contain grit! I went straight to the pet store, bought some grit. Saturn went wild for the grit. The other two had a little bit, not much.
Then Saturn attempted to poo. It got stuck, so I gently pulled it out with paper towel. There was a bit of hair/plastic or something that had made it stuck. It came out OK and she even pooed by herself (normal poos) after this.
She is sleeping like a dream tonight. Cuddled up with the other chicks. No distressed chirps, just happy chatter.
Thanks everyone for your support. And thank you Wyorp Rock - your advice made me double check and find the problem!!
It's such a relief to see her happy and it warms my heart to see her eating, pooping and sleeping soundly!